Known in the past has been an exhaust purification system of an internal combustion engine, which diagnoses an exhaust purification catalyst for an abnormality (degree of deterioration), based on an output of an NOX sensor arranged at a downstream side of the exhaust purification catalyst in the direction of flow of exhaust. As such an exhaust purification system, for example, there is known one which judges that the exhaust purification catalyst is abnormal when the output of the NOX sensor when the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst is near the stoichiometric air-fuel ratio is a reference value or more (for example, PLT 1).